The present invention is directed to a composition for blocking light and a method of using the composition. More specifically, the present invention is directed to a composition for blocking light, which is dispersible or soluble in aqueous base, retains its shape after application and may be used to form an image on a substrate.
Methods of forming images on substrates encompass various industries such as the electronics, graphic arts and textile industries. Forming images typically involves lithography or photolithography. For example printed fabric labels may be made using a variety of techniques, such as screen printing, offset lithography printing, dyeing, flexographic printing, in-plant printing, and transfer printing. Such labels are suitable for garments for the purpose of decoration, identification, advertising, wash and care instructions, size, price, as well as other purposes.
Screen printing, also known as silk screen, employs a porous stencil mounted on a screen, in which the non-printing areas are protected by the stencil. The masking material also may be dried lacquer, shellac or glue. Printing is done on a mechanized press by feeding cloth under the screen, applying ink with a paint-like consistency to the screen, and spreading and forcing it through the fine mesh openings with a squeegee.
In offset lithography methods, the image and non-image areas are essentially on the same plane of the surface of a thin metal plate, the definition between them being maintained chemically. The ink is picked up by the hydrophobic areas on the plate but is not picked up by the hydrophilic areas. The image is then transferred to an offset rubber roll, then from the roll to the fabric sheet.
Flexographic printing is a form of rotary letterpress using flexible rubber plates and fast-drying fluid inks. The rubber plates utilize the relief method for image creation, where the image area is raised above the non-image areas. Ink rollers only touch the top surface of the raised area. The surrounding, non-printing, areas are lower and do not receive ink. The inked image is transferred directly to a cloth. Dyeing can be achieved by using dyestuffs rather than pigmented inks in any of the printing processes described above. The use of dyes, however, requires additional after treatments to fix the dye in the fabric.
In the electronics industry images are formed on substrates for the formation of circuitry by photolithography. This involves the use of a radiant energy sensitive material, such as photosensitive material, that is applied to a surface as a whole area coating (spin-casting, roller coating, spray and screen printing, and dipping) or as a whole area sheet (lamination). The material is applied in a light controlled laboratory in order to ensure that the photosensitive material is not pre-exposed prior to introducing the required pattern mask in front of the coated wafer or copper panels. The mask can be either a contact mask, a proximity mask or a projection mask. In all cases the mask is manufactured as a discrete unit to a high precision and is carefully protected against damage or dust/particulate collection. Once the mask is put in place then a lamp, of a radiation material matched to a photoinitiator used in the photosensitive material, may be used to expose the substrate coating in those areas not protected by the mask. Depending upon the photosensitive material type employed the pattern transfer achieved may be either positive or negative with respect to the mask. After exposure the photosensitive material is exposed to a developing chemical that modifies the chemistry of the coating in such a manner as to permit the untreated material to be washed away in a water-based dip bath or conveyor shower/spray.
Although spin-cast, dip, roller coating, spray and screen printing, or sheet lamination photolithographic methods of achieving a surface relief image are successful, they do have a number of problems such as material wastage (because of whole area technique), selective 3-D patterning is difficult and time consuming, chemistry used in photosensitive material has a high toxicity rating, disposability of large volumes of toxic and developing chemicals, and simple patterning is a multiple step process such as photocoating, mask alignment, radiation exposure, mask removal, pattern development, excess material rinse removal, and substrate drying.
One or more of these problems may be addressed by introducing further processes that can provide a patterned relief structure on a surface, including stenciling (screen printing), microdot transfer (stamping), and laser writing-etching (includes ablation scribing and direct-write photolithography equivalent imaging). Each technique has its merits and limitations which are driven by the detail of the intended application such as speed pattern generations, relief pattern thickness, controlled etch capability, cost of process and ease of use process. However, any one process does not address all of the problems cited above.
U.S. Pat. No. 6,093,239 discloses hot melt inks, which may be used to form a mark on a substrate, such as in printing screen manufacturing processes. The patent discloses that the inks are at least 50% water auto-dispersible. The inks contain waxes or resins, colorants, stabilizers and may be applied using an ink jet apparatus. The patent discloses that the inks are solid at room temperature and liquid at temperatures above room temperature. The patent also discloses that the inks may be readily removed from a substrate with water after imaging is completed. However, the water auto-dispersibility of the inks is one disadvantage. Such inks are not suitable for use in humid environments such as may be found in many manufacturing plants. Such inks may readily become too fluid due to moisture absorption for proper application.
Many methods used in the manufacture of electronic devices require selective application of a photosensitive material, which is then used to enable subsequent steps of the overall manufacturing process. For example, solder mask is excluded from through-holes in a printed wiring board but is present in other areas of the board which require resistance to solder applied later in the manufacturing process.
A variety of methods are currently practiced that enable the selective final presence of solder mask or other photosensitive material. For example, solder mask is patterned to fully cover electronic circuitry except for those portions intended to be exposed, e.g., for soldering to another component. Solder masks are typically formed from photosensitive material which is applied to a substrate such as a printed circuit board. The photosensitive material is exposed to actinic radiation, which is imaged by means of an artwork or phototool. Subsequent to exposure, the photosensitive material is developed in a solvent which washes away either exposed or unexposed portions of the material (depending upon whether the photosensitive material is positive-acting or negative-acting). The portion of the material which remains on the substrate is then cured, e.g., with heat or UV light to form a hard, permanent solder mask intended to protect the printed circuitry.
One problem in the electronics industry is proper alignment or registration such as in the manufacture of multi-layer printed wiring boards. Registration is the relative position of one or more printed wiring patterns or portions thereof with respect to desired locations on a printed wiring board or another pattern on the other side of the board. One of the challenges in the manufacture of multi-layer printed wiring boards is to obtain adequate innerlayer registration. Internal features must be registered accurately to each other, and they must be accurately registered to any drilled holes. Hole-to-innerlayer misregistration creates two potential reliability problems: failure of the hole to line connection and shorts between holes and isolated conductors. Misregistration of internal layers also increases electrical resistance and decreases conductivity. Severe misregitration creates an open-circuit condition, a complete loss of continuity.
One of the last steps in the manufacture of multi-layer printed wiring boards is the application of the solder mask onto an outside layer. As mentioned above the solder mask is selectively exposed using a phototool such that specific areas can be developed off of the board. Such phototools, typically composed of diazo, silver halide or quartz and chrome, are prepared based on “idealized” dimensions of circuit line placement. However, variations in actual board dimensions of the circuit line from the “idealized” dimensions are common because of rigorous processing employed in the manufacture of the boards. Using an “idealized” phototool in combination with dynamically changing boards often results in registration problems between boards in a multi-layer laminate. Because the solder mask step is one of the last steps in the manufacture of multi-layer printed wiring boards, discarded boards caused by misregistration lead to costly and inefficient manufacturing processes.
Further, in conventional practice workers often prepare multiple fixed phototools and manually try to find the optimum fit between phototool and board to avoid misregistration. Such a process is both inaccurate and time consuming resulting in further inefficiency of multi-layer printed wiring board manufacture.
Accordingly, there is a need for improved methods of forming images on substrates.